As data processing activities become ever more important to our society, the reward for subverting these activities evidently grows proportionally in the mind of vandals who are both technically proficient and socially dysfunctional. Although the number of such vandals is relatively small, they do untold damage by spreading computer viruses, altering records, obliterating patiently collected databases, and so forth.
As a result, a number of useful tools have been developed to combat electronic vandalism. A recent advance by toolmakers is their development of vulnerability scanners. Vulnerability scanners probe a data processing system such as a host computer or an Internet server to uncover externally visible vulnerabilities, i.e., security vulnerabilities that can be detected and therefore exploited by someone interacting with the data processing system from the outside.
Scanners probe according to known vulnerabilities, for example those listed in the Common Vulnerabilities and Exposures list sponsored by MITRE Corporation. Known vulnerabilities may include server misconfigurations, buffer-overflow problems that make operating systems vulnerable to denial-of-service attacks, insecurities that cause operating systems to respond to vandals' prodding in ways that subtly identify the operating system's release level and thereby reveal its entire spectrum of vulnerabilities, and so forth.
When the vulnerability scanner determines that a data processing system has a particular vulnerability, the scanner reports that vulnerability to an administrator. Once so informed by the vulnerability scanner, the administrator may take preventive action, for example by installing an operating-system patch, by reconfiguring an improperly configured server, and so forth. Thus the information provided by the vulnerability scanner is invaluable in ferreting out externally visible vulnerabilities, so that they may be eliminated. Nevertheless, the data processing system continues to be at risk until the administrator actually takes the required action to eliminate the externally visible vulnerability.
Unfortunately, a vandal as well as an administrator may exploit the power of a vulnerability scanner to ferret out weakness in a data processing system. For example, the vandal may use the same scanner as that used by the administrator, identify exactly the same externally visible vulnerability as that identified by the administrator, and with benefit of this knowledge attack the data processing system on its weakest front. Because of the scanner's power to find externally visible vulnerabilities, the vandal's attack is highly likely to succeed if it is launched before the administrator acts to eliminate the vulnerability.
So, in a logical sense, tool makers and vandals play a game: an advance of one camp is countered by an advance of the other. Here, the toolmaker's legitimate advance—the vulnerability scanner—plays as well into the illegitimate hands of the vandal. Consequently, there is a need to protect a data processing system such as an Internet network server from attacks by vandals who use vulnerability scanners to identify the data processing system's externally visible vulnerabilities.